In Mail.app I have a number of rules that reroute incoming mail to certain folders. The messages sorted into those folders are not particularly important, so I don't like hearing the new mail sound unless the message makes it to my inbox.

I looked at the various actions one can perform for a rule, but there was no option to 'play no sound.' Solution? Open Audacity and generate a one second, 25,000Hz tone and export it as an AIFF to ~/Library/Sounds/. Next add a play sound rule and select the tone (nomailsilence25kHz.aiff seemed logical to me).

Why not just generate one second of silence? Well, Audacity doesn't let you export a completely silent file. 25,000Hz is a frequency well above the human hearing range. Unfortunately, your pets will still receive the new mail notifications.

[crarko adds: I haven't tested this one. I can just imagine the reaction of my dogs to this.]

I doubt the sound hardware in your Mac will reproduce 25 kHz at any audible volume, even to dogs. But there are plenty of other ways of generating a silent sound file. In fact, if you Google 'silent mp3' you'll find plenty for download.

There are many programs that will allow you to create a silent file. GarageBand should do the trick... Audio Hijack Pro, Logic, and probably some test-tone generators would work too.

Also - don't worry, your dogs won't hear a thing because your computer is outputting with a maximum samplerate of 48kHz. Anything above half of that (ie above 24kHz) is filtered out to negate the effects of aliasing.

Most speakers will probably cut off well before 20 kHz. In addition, lots of digitized audio has already been pre-filtered before sampling at either 44.1 kHz or 48 kHz (for high-quality encoding). A typical response range for stereo computer speakers is 80 Hz - 15 kHz.

FWIW, you can test both your speakers and your own ears here: http://www.freemosquitoringtones.org

That's what I've been doing for years. Standard sound switched off. Different rules play different sounds (or none at all) and stop applying further rules. A default rule at the end plays a sound for all remaining messages. Works perfectly.

It just occurred to me that this alternative might play the final rule sound for every email that is left in the inbox, yuck. Unless Mail is smart enough to group filter actions that happen close together and play the sound once (though a large email might still break that grouping).

In my case, I set up an Applescript to *speak* when I get a mail: it says "Mail from" and then the sender of the email. Jolly useful: I can hear immediately how urgent it's likely to be, and decide whether to stop what I'm doing.

My mail.app both on my macbook pro and on my current generation iMac has mail notification sounds that are as unreliable as an M16 in the mud. I'd say I get a new mail notification sound like 15% of the time.

I strongly suspect this is some sort of interaction between mobileme and my iPhone/Desktops.